For some time, it has been customary practice to bend tubing with the use of an internal mandrel so as to minimize wrinkling or crimping of the tube as it is bent; and, while the use of internal mandrels is accepted commercial practice for larger-sized tubing in excess of 3" in diameter, has not been found satisfactory in the bending of smaller tubing.
U.S. Letters Pat. No. 4,130,004 to H. L. Eaton decribes various approaches which have been followed in the bending of tubing without the use of a mandrel, particularly those bending operations which employ compression or draw bending, or a combination of both, in an effort to overcome undesirable wrinkling or buckling of the inner wall or side of the tube as it is being bent around a bend die. Eaton proposes utilization of a generally heart-shaped die and initiates the bending operation by causing a pressure die initially to operate in a draw bending mode followed by a compression bending mode with the tube clamped between the bend die and a clamping die. The pressure die serves to force the tube against the bending die and initially to advance with the tube so as to exert little or no axial restraint in the initial bending then to apply increasing pressure while interrupting the movement of the pressure die so as to exert increasing axial restraint upon the tube in compression bending to complete the bending operation.
Other tube bending operations propose utilization of multiple radius grooves both in the bending and pressure die grooves in order to minimize wrinkling or distortion of the tube through the bending operation and, for example, in U.S. Letters Pat. No. 2,986,195 to A. R. Landis the forming grooves have cross-sectional radii less than one-half the minimum allowable outside diameter of the tube to be bent and which extend around less than 180.degree. of the bending die. In the bending operation, Landis is concerned with avoiding any relative axial shearing movement between the forming dies.
In U.S. Letters Pat. No. 2,955,638 to A. Hellwig, a multiple radius groove is formed on the bending die having an undersized semi-circular groove to impart a squeezing action to the tube in combination with a tangential extension of limited length on the bend die.
Notwithstanding the various approaches taken to bending of tubing and pipe through the use of multiple radius tubing, none to the best of our knowledge has successfully achieved bending of tubing and pipe over different size ranges in such a way as to substantially minimize to the point of eliminating undesirable bulging, collapse or wrinkling of the tube while maintaining a substantially uniform diameter throughout. It has been found that to achieve successful bending of tubing and pipe necessitates the coordination of the relative pressure and speed of advancement between a bending die and pressure die along with a predetermined degree of pressure against the tube and close control over the change in radius between the confronting grooves of the bend die and pressure die to minimize any distortion or wrinkling that is customarily experienced in the methods and apparatus presently in use.